1. Field of the Invention
The present invention relates to an image fixing apparatus for use in an electrophotographic copying machine, more particularly to an image fixing apparatus for thermally fixing toner images on a transfer sheet. The present invention also relates to an image fixing roller for use in the image fixing apparatus.
2. Discussion of Background
For example, in a conventional electrophotographic copying machine provided with a laser printer, a rotatable photoconductor drum is provided, and copies are made with the following steps: A photoconductive portion of the photoconductive drum is uniformly charged by a charging unit, and information is recorded in the form of latent electrostatic images by the application of a laser beam thereto by a laser scanning unit. The latent electrostatic images are then developed with toner to toner images by a development unit in the electrophotographic copying machine. The developed toner images are then transferred to a recording sheet. The toner-images-bearing recording sheet is then passed through a thermal image fixing apparatus, in which the toner images are thermally fixed to the recording sheet. Thus, copies are made by the conventional electrophotographic copying machine.
In the above-mentioned conventional thermal image fixing apparatus, for instance, an image fixing roller as illustrated in FIG. 10 is employed, which is composed of a hollow core cylinder 21 which is made of, for instance, aluminum, and a toner-releasing layer 22 which is made of, for instance, a fluoroplastic, and provided on the outer peripheral surface of the hollow core cylinder 21. The toner-releasing layer 22 is capable of preventing toner from adhering to the outer peripheral surface of the image fixing roller during the image fixing process, and releasing toner from the surface of the image fixing roller.
In the image fixing roller, a heater (not shown) such as a halogen lamp is provided in a vacant portion within the hollow core cylinder 21 along the revolution axis thereof, whereby the image fixing roller is heated from the inside thereof by the radiation heat from the heater.
In parallel with the image fixing roller, there is provided a pressure application roller (not shown) which comes into pressure contact with the peripheral surface of the image fixing roller. The image fixing roller and the pressure application roller are rotated in the same direction in the contact portion where the two rollers are mutually in pressure contact, and the toner-images-bearing recording sheet is transported so as to pass through the contact portion between the two rollers, whereby the toner images transferred to the recording sheet are softened by the heat from the image fixing roller and fixed to the recording sheet which is held between the two rollers, under the application of the pressure thereto by the pressure application roller.
In such a thermal image fixing apparatus, however, a relatively long warm-up time is required before the outer peripheral surface of the image fixing roller reaches a predetermined image fixing temperature required for toner image fixing after the thermal image fixing apparatus is powered.
Conventionally, in order to shorten the warm-up time, the main switch for the image fixing apparatus is designed in such a manner that when turned on, the preheating of the image fixing roller is started and continued. This method, however, has the shortcoming of wasting a significant amount of power.
Further, in order to avoid the above problem, there have been proposed, for example, the following various methods for shortening the worm-up time for such an image fixing roller:
A method of providing a resistive heat emitting layer at or near the peripheral surface of an image fixing roller (Japanese Laid-Open Patent Applications 55-164860, 56-138766 and 2-285383); a method of blackening the inner wall of a hollow portion of an image fixing roller to increase the radiant efficiency thereof, thereby increasing the heat absorption efficiency, and a method of increasing the surface area of the inner wall of a hollow portion of an image fixing roller by roughening the surface of the inner wall (Japanese Laid-Open Patent Applications 4-34483 and 4-134387); a method of constructing an image fixing roller composed of a heat pipe (Japanese Laid-Open Patent Application 3-139684); a method of heating an image fixing roller by electromagnetic induction (Japanese Patent Laid-Open Application 4-55055); a method of constructing an image fixing roller by use of an electroconductive elastic material and causing electric current to flow therethrough, thereby directly heating the image fixing roller (Japanese Laid-Open Patent Application 4-186270); and a method of constructing an image fixing roller which includes a cylindrical heater in which a positive thermistor material is used (Japanese Laid-Open Patent Application 4-42185).
In order to make the above-mentioned methods actually effective in practical use, it is required that the core roller for each of the image fixing rollers have good heat conductivity. However, there is a limitation to the reduction of the thickness of the core roller for increasing the heat conductivity in view of the mechanical strength required for the image fixing roller for use in practice. Therefore the above-mentioned methods are not always practical. Furthermore, a large amount of energy has to be applied to the heating elements such as heaters for the image fixing rollers in order to sufficiently shorten the warm-up time for such conventional image fixing rollers.